x=int(input())
if x<999 or x>10000:
    print ("0")
    exit ()
M=0
x1=x
x2=x//10
x3=x//100
x4=x//1000
x5=x%1000
x6=x%1000//10
x7=x%1000//100
x8=x%100
x9=x%100//10
x10=x%10
S=[x1,x2,x3,x4,x5,x6,x7,x8,x9,x10]
for y in S:
    k=0
    for i in range(1,y//2+1):
        if y%i==0:
            k+=1
    if k==1:
        M=max(M,y)
print (M)
